That not so fresh feeling

If I were creating a caching proxy, I think I would have it tune by assuming the content needed to be refreshed frequently. Only after a history is established should it save bandwidth by checking for updates less frequently.
We implemented a transparent caching proxy this week. I’m seeing cache freshness issues. When I talk to the vendor about this they blame bad websites. Most websites, use HTTP headers to indicate when the content expires. The problem with this response, is you have to take the Internet as it comes to you. Without this proxy, my users are just fine. Adding the proxy is supposed to make things work better not worse. In my opinion its the vendors responsibility to make it work.
So I’m left with promises that as I add more users, and time passes, the proxies freshness algorithms will learn and I wont see these issues. The vendor points out they have 70% of the caching market so they must be good. I’m left looking at yesterdays news.